This is a device that outputs a pulse whenever the input voltage it is monitoring rises above or falls below certain (typically user-definable) thresholds. When the input is greater than the threshold, the output would be high. Similarly, if it is below a certain threshold, the output is low. If the two thresholds are different, and the the input falls between the two values, the output remains unchanged. The Schmitt Trigger was invented by an American scientist, Otto H. Schmitt, in 1934 while he was still a graduate student. In his doctoral dissertation in 1937, he discribed it as a “thermionic trigger.” Oddly enough, this came about as a direct result of Schmitt’s study of nerve propogation in squids, making it possibly the oddest link between audio and biology in history.